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Cognitive Neuroscience Lecture 3: Cognitive Neuroscience Methods
Where is the "seat of the mind"? * Is it the heart? The ventricles? People didn't used to know. * Phrenology: '''different functions localized to different regions ** looked at size of the bumps on skull to deduce which areas are highly developed in an individual Against phrenology: anti-localizationist * '''Fluorens destroyed parts of pigeon brain and noticed that many faculties affected by one part, and that they recovered; concluded that all parts are capable of all functions * This was evidence for the aggregate field theory: all sensations, all perceptions and all volitions occupy the same seat in the cerebral organs. The faculty of sensation, percept and volition is then essentially one faculty * Noticed that many functions affected by removing parts Big debate: Localization (specific function maps to specific part) vs. equipotentiality '''(all parts of brain are capable of all functions) * skull palpators (phrenology/localization) vs. bird-brain ablators (Fluorens/equipotentiality) * Phrenology is a case study in bad science (pseudoscience), but the idea of localization of function is important * They really just needed the right methods to study! Namely... * '''Neurology and neuropsychology: looking at diseased/damaged brain Modern Neuropsychology * John Hughlings Jackson: noticed characteristic progression of seizures, which led him to propose a topographic organization in cerebral cortex (e.g. map of body) ** his logic was that if a seizure progresses in a particular way across the body, a - later confirmed to be true - less extreme type of localization o localization of symptoms or behavior, '' not ''function o based on interconnectedness of brain areas - Claude Bernard: same mentality; you need to look at the body as a whole Paul Broca: '''patient could comprehend speech but not produce - “tan, tan, tan, tan” - Left inferior frontal lobe damage: Broca’s area '''Carl Wernicke: patient could produce speech (fluent aphasia), but not comprehend - Nonsensical speech - Cannot understand written language - “I called my mother on the television and did not understand the door…” - Left posterior temporal/parietal region Broca’s area: speech production Wernicke’s area: language comprehension Double dissociation: language comprehension and speech production are dissociated; selective impairment in one but not other Causes of neurological/neuropsychological problems: - vascular disorders: stroke, hemorrhage o tissue supplied by damaged artery dies - Tumor: usually in white matter - Degenerative disorders: o Alzheimer’s (plaques and tangles) § Above average brain volume shrinkage § Especially shrinkage in hippocampus (hippocampus—memory) § Brain generation should develop treatments because Alzheimer’s is 3x cost of other elderly diseases o Parkinson’s (loss of dopamine neurons) § Loss of coordination o Huntington’s (basal ganglia atrophy) o Pick’s (fronto-temporal atrophy) § Difficulty in speech/thinking § Personality change - Viral infection, nutritional deficiency o AIDS destroys white matter Studying Patients with brain lesions - Look at diseased/damaged brains - Assess mental abilities through cognitive testing - Attempts to link specific cognitive deficits to damage in specific ''brain regions - If a specific brain region contributes to a particular cognitive task, then a lesion in that area should impair performance of that task - Example: Is ______ function dependent on the same brain structure as _________ function? - '''Single dissociations': Patient A can do Task X but not Y o E.g. can name something using smell but not sight - Problem with single dissociation: one task may be more sensitive than the other task because of difficulty/sensitivity - For example, a task that measures something might need more concentration than the other task o Performance may just reflect generalized concentration problem in patients, not specifically what you’re looking for (memory, familiarity, etc) o Performance may be bad because you’re looking at patients, a group likely to already have trouble with difficult or concentration-requiring tasks - Double dissociations: Patient A can do Task X but not Y ' '''Patient B can do Task Y but not X o Perform 2 memory tests; you don’t need them to be in the correct order. o Then, you’ve controlled for any cognitive setback that is generalized to all neurological patients (e.g. difficulty with concentration) Why double dissoc? Consider the following single dissoc: Damage A à can’t read à conclude that A critical for reading Is it that they just can’t read, or is it a broad perceptual problem? Maybe A is not just for reading, but for perception in general. You can check whether it is just for perception in general by doing other perceptual tasks, but that will only show that A is not specific to reading…it won’t show what IS specific to reading. So if you do something where a person with damage A can, let’s say, recognize images (another perceptual task) but not read, then you’ve shown that A is not just generally for perception, but is for reading specifically. Advantages & Disadvantages of studying Patients with Brain Lesions Advantages: - Natural experiment - Suggests causal role for brain area in particular cognitive function - Double dissociations provide strong evidence for separable processes o E.g. Broca’s vs. Wernicke’s, production vs. comprehension Disadvantages: - Somebody gets brain damage - Not controlled experiment - Damage not always localized; many functions can be impaired - Damage not same from person to person - Plasticity, recovery, connections to other regions (dynamic processes you can’t control) Cognitive Neuroscience Methods Disruption methods: '''what function does certain region have' - Isolate brain area with disruption - Brain disruption implies cognitive disruption - Measure task performance - Determine which cognitive function is supported by the disrupted brain area - They cannot do ________ when _____ region is disrupted Recording methods: what area lets me do certain task - Isolate a cognitive function through task selection (task involves _____ cognitive function) - Engage the brain in that function - Measure the brain activity while it is performing that function - Determine which brain area supports the targeted cognitive function - They must use ______ region in order to do ______ Cognitive Psychology - Study of mental activity as an information processing problem - Cognitive psychology theories refer to hypothetical mechanisms: underlying mental processes and representations - Try to measure in terms of % or reaction time. Why? - Because we want to know WHAT FACTORS CHANGE how fast or accurate you are, and we want to CONTROL them o Measure/control 1 variable at a time Testing mental representations from behavioral methods - Letter matching-task: press if two letters are from same category (vowel/consonant) or different - Different response time: degree of mental processing required to perform the letter-matching task - Identical (e.g. AA or aa) has faster response time than just same letter (e.g. Aa) o This implies that letters are represented with physical identity, not just phonetic Disruption Methods - Permanent lesions - Temporary disruption o Pharmacology o Cortical stimulation o TMS (trans-cranial magnetic stimulation) Cortical stimulation: Case study AA - Experienced seizures at age 9 - Tumor discovered and removed - Seizures and tumor returned at 16 - Cortical stimulation undertaken prior to tumor resection - Subdural electrode array placed on cortex surface - Allows for recording (to identify epileptic focus) and stimulation - Stimulation can produce disruption of cognitive processes supported by underlying neural tissue o This is used by surgeons to limit cognitive impact of surgery (feedback as they go along) - Research as well: cortical stim can help understand brain-cognition relationships o o What is the empirical logic? What assumptions must we make? o Logic: disrupted region controls impaired task. o Assumption: we assume that the task impairment is not a result of a generalized impairment in concentration or other larger cognitive ability that is disrupted in all surgical patients. This is a single dissociation study, not double dissociation, so we have not controlled for it being a neurosurgical patient. - Procedure on each trial: o Stim and non-stim trials mixed o Stim and non-stim don’t feel different o On each trial: some amt of electricity for 5 sec o Stim presented 1 sec into period o Asked to respond ASAP - Results: lower percent correct during stim of region A - Can you make the conclusion that A is the multiplication center? o No, because it could be necessary for number recognition. - Empirical logic: cortical stimulation can produce disruption of cognitive processes subserved by underlying neural tissue o This does not mean that this tissue is solely responsible for that process o I would want to see him do addition and other tasks with numbers, as well as letters to remove confounding variables o Conclusion that it is the “multiplication center” is unwarranted because it could be for memorization, symbol recognition o - Additional data provided was language production, picture naming, reading aloud, responding to commands, all of which got above 90%. Also, stim on that area affected math, stim on other areas did not o This implies that it really is math related…but I’d still want to see more about numbers The challenge in Cognitive Neuroscience - To learn about brain/cognition, we ask participants to perform tasks - But a task typically involves multiple cognitive functions/mental operations - We are interested in identifying the neural substrates of specific cognitive functions, not the entire task - Challenge: isolating a specific function from within a complex task Doing both of these processes would isolate multiplication from addition - We do experiment with stim and no stim and in each, test multiplication and addition, and measure response time and % correct in each TMS (trans-cranial magnetic stimulation): Disruption method - Magnetic stimulation can disrupt normal activity of the brain, creating a “virtual lesion” - Magnetic field outside the skull induces electric field inside skull o Tight wire coil insulated o Electrical current thru coil o Magnetic field generated that goes through skin, scalp and generates current o Neurons fire - MRI can be used to display cortical area targeted - Direct effects are about 1-1.5 cm^3 o Activation restricted to this size - There are different coil types, and different induced fields - Single pulse or repetitive TMS (rTMS) - rTMS: longer lasting effect, used in research and also effective as a therapy for depression - if a TMS is placed over the hand area of the motor cortex, muscles of the wrist and fingers will be activated o twitches o participant knows it’s involuntary - subject begins counting and then Broca’s area (left) is stim - when people temporarily given a pulse, they failed to identify what they were supposed to identify - it takes time to perceive and analyze letter. o Same response time as if it was destroyed TMS advantages - higher spatial resolution than neuropsych - changes neural activity in a spherical area of 1 cm^3 - high temporal resolution o when the pulse follows the stimulus by a certain amt of time, participant fails to ID stim on large % of trials - can be used repeatedly - provides info about causal role of brain area in particular perceptual/cognitive/motor function - non-invasive TMS disadvantages - restricted to brain regions close to the skull - difficult to hold the coil in place - has induced seizures in some epileptic patients Recording brain activity - invasive, measure electrical activity o single cell recording in animal - non-invasive, measure blood flow or electrical activity o PET (positron emission tomography) o fMRI (functional magnetic resonance imaging) o ERP (event related potentials) o MEG (magneto-encephalography) Single or multi unit recording in animal - Measure the firing rate '''of a single cell by inserting an electrode either inside a cell or just outside (intra or extracellular) - Determine which experimental manipulations produce a consistent change in the baseline response rate of cell - If cell is responsive to some stim characteristics and not others, the inference is that the cell is involved in the representation/processing of that feature Receptive field - While activity of single cell is monitored, monkey is required to maintain fixation (possibly anesthetize; cats) - Bar of light is presented at various positions in FoV until stim and location are found that produce a response in the neuron being recorded - '''Receptive field: the location over which the stimulation affects the response of the neuron - More of stim in receptive field = more firing Advantages of single/multi unit recording in animal - Single cells can be highly selective in terms of response - Very high temporal and spatial res - Newer multi-unit recording techniques allow researchers to record from several hundred cells simultaneously Disadvantages of single/multiunit recording in animal - Invasive - Fishing expedition: cannot test all possibilities, so your choice of what to test limits possible outcomes of experiments - Sampling a very small percentage of neurons - Provides info about relationship between task and neuronal activity but not about causal role of neuron in cognitive task (i.e. we don’t know what would happen without this type of neuron, just that this neuron exhibits activity under certain conditions)